Abstract: Abstract: Urea has the same as the liquid ammonia denitration performance and the use of safe. The project of replacing liquid ammonia with urea is being carried out in thermal power. However, in operation, the use of hot primary air with large dust content as ammonia injection dilution wind will lead to the blockage of ammonia injection pipeline, resulting in the increase of ammonia injection volume and ammonia escape, and the serious blockage of air preheater. This paper takes a power plant as an example to reform the dilution air system and achieves good results.